The present invention relates to a liquid phase sintered body of a carbonitride alloy with titanium as main component which alloy has improved properties particularly when used as a cutting tool material in cutting operations requiring high thermal shock resistance. These improved properties have been achieved by processing the material in a specific way to obtain a lower melting point of the liquid phase in the interior of the body compared to the surface. In this way the porosity and residual oxygen content are minimized and, in addition, a binder phase gradient leading to a beneficial compressive residual stress in the surface zone can be produced.
Titanium-based carbonitride alloys, so called cermets, are today well established as insert material in the metal cutting industry and are especially used for finishing. They comprise carbonitride hard constituents embedded in a metallic binder phase. The hard constituent grains generally have a complex structure with a core surrounded by a rim of other composition.
In addition to titanium, group VIa elements, normally both molybdenum and tungsten and sometimes chromium, are added to facilitate wetting between binder and hard constituents and to strengthen the binder by means of solution hardening. Group IVa and/or Va elements, i.e., Zr, Hf, V, Nb and Ta, are also added in all commercial alloys available today. All these additional elements are usually added as carbides, nitrides and/or carbonitrides. The grain size of the hard constituents is usually &lt;2 .mu.m. The binder phase is normally a solid solution of mainly both cobalt and nickel. The amount of binder phase is generally 3-25 wt %. Other elements are sometimes added as well, e.g., aluminum, which are said to harden the binder phase and/or improve the wetting between hard constituents and binder phase.
Despite more than 20 years of intense development efforts world-wide, it has not heretofore been possible to increase the rather narrow application area of cermets. They are still limited to finishing or at most semi-finishing operations requiring moderate wear resistance and toughness. U.S. Pat. No. 4,985,070 discloses a process for producing a high strength cermet by sintering the material in progressively increasing nitrogen partial pressure to eliminate denitrification and obtain better control of the final nitrogen content. This is useful to obtain improved process control during conventional sintering especially of cermets with extremely high nitrogen content. Unfortunately, it also eliminates the possibility of producing different melting points in different parts of the material, the process utilized in the present invention.
Through WO 96/22403, which corresponds to U.S. Ser. No. 08/875,139, and WO 97/04143, which corresponds to U.S. Ser. No. 08/981,844, both references herein incorporated by reference, the limited wear resistance of cermets has been overcome and a significant step forward has been taken towards tougher material. This is accomplished by optimizing the raw material compositions and by applying CVD coatings onto suitable cermet alloys to obtain compressive residual stresses in the coating which increases toughness. In both cases, a conventional sintering technique was used. However, further steps toward improved toughness, in particular improved thermal shock resistance, need to be taken to compete with CVD-coated WC-Co based alloys in many applications demanding toughness. When doing so, it is most probably necessary to find novel processing methods. Continued optimization mainly of chemical composition and raw material compositions is not likely to have the desired effect.